The objective of this project is to clarify the concept of motivation to master the inanimate environment. Forty-four one year old infants were observed in three situations: free play, a test of cognitive functioning, and situations designed to measure mastery motivation. For the study of mastery motivation, tasks were developed to elicit: attempts to solve difficult problems, the practice of emerging skills, and the production of effects. We found high consistency between measures of persistence on conceptually similar tasks administered in different settings, but only modest interrelations among persistence measures on the three different types of mastery tasks. We found a number of significant relations between the early environment and mastery motivation at one year. There were significant relationships between kinesthetic stimulation at 6 months and the infant's persistence in attempting to elicit feedback from objects, his persistence on problem solving tasks and his competence on these tasks. These same aspects of mastery behavior were also related to the responsiveness to the infant's distress, to his persistence in practicing emerging skills and to this competence on these tasks. BIBLIOGRAPHIC REFERENCES: Yarrow, L. J., Morgan, G. A., Jennings, K. D., Harmon, R. J., Gaiter, J. L., and Vietze, P. M. Mastery motivation: A concept in need of measures. Educational Resources Information Center, ED 129 415, 1977. Morgan, G. A., Harmon, R. J., Gaiter, J. L., Jennings, K. D., Gist, N. F. and Yarrow, L. J. A method for assessing mastery in one-year infants. JSAS Catalog of Selected Documents in Psychology, 1977.